The invention relates to a colour display tube comprising in an evacuated envelope an electron gun system for producing three electron beams directed onto a display window. For each electron beam the electron gun system is provided with a cathode which is secured in a cup-shaped apertured control grid by means of an electrically insulating material. The electron gun system further includes a common anode having a respective aperture for each electron beam and means to focus the electron beams on the display window.
Such a colour display tube is known from Netherlands Pat. application No. 7402421 which corresponds to U.S. Pat. No. 3,973,161. This tube comprises an electron gun system which is provided with a separate cathode for each of three electron beams. Each cathode is secured in a cup-shaped, apertured control grid by means of a ring of ceramic material. The anode is formed by a plate which is common for the three electron beams and has an aperture for each electron beam. The means to focus the electron beams on the display window are formed by two electrodes which are common for the three electron beams.
In such a tube the electrodes of the electron gun system should be positioned accurately with respect to each other. This applies in particular to the mutual position of the control grids and the centering of the aperture in each of the control grids relative to the corresponding aperture in the anode. During assembly of the electron gun system, the electrodes are positioned relative to each other in an assembly jig. The position of the electrodes is fixed by means of suspension braces connected thereto, the ends of which are sealed in glass rods. As a result of the heating up of the suspension braces and the electrodes when the glass rods are depressed, deviations easily occur in the adjusted position of the electrodes. Moreover, the adjusted position of the electrodes varies as a result of expansion of the suspension braces and of the electrodes during warming up of the tube during operation, which results in a variation of the convergence of the three electron beams on the display window.